A central question in cell biology concerns how cells destroy their own proteins. Protein misfolding represents a universal threat to cells, and has numerous causes including heat, errors in translation, DNA damage, heavy metals and metalloids, inherited mutation, oxidation, and aging. Protein misfolding is also associated with most neurodegenerative disease and some cancer types. The ubiquitin-proteasome system, by virtue of its ability to destroy proteins, responds to the threat of protein misfolding, sometimes termed proteotoxicity. We have identified a novel arm of this stress response pathway that specifically protects cells from misfolded proteins induced by trivalent metalloids like arsenic, bt not other causes of protein misfolding. The key mediators of this response pathway are Cuz1, and the related but largely uncharacterized protein, Tmc1. We showed that Cuz1 functions as a novel zinc-dependent ubiquitin binding protein that interacts with the proteasome and the multifunctional chaperone Cdc48/p97. These data suggest a model in which Cuz1 protects cells from trivalent metalloids by recognizing misfolded proteins and delivering them to the proteasome for destruction. The long-term goal of my work is to provide a better understanding of how intracellular protein degradation occurs. In this application, I use a combination of biochemical, genetic, cell biologic, proteomic, and structural approaches to understand the stress response pathway mediated by Cuz1 and Tmc1. First, I will characterize the ubiquitin binding function of Cuz1. My data indicate a potentially novel evolutionarily conserved ubiquitin recognition motif within Cuz1. If verified, Cuz1 might represent the founding member of a new class of ubiquitin binding proteins. Second, I will utilize proteomics approaches to identify specific substrates of Cuz1. Because Cuz1 is remarkably specific in protecting cells from metalloid-induced proteotoxicity, these studies may provide insight into one of the most important questions in protein degradation, which relates to how specificity is generated and maintained. Third, we will undertake the first significant characterization of Tmc1, a zinc finger protein with sequence and phenotypic similarity to Cuz1. I will determine how Cuz1 and Tmc1 function in coordinating their protective stress response. Together, these aims will provide insight into a fundamental aspect of cell biology, regulated protein degradation, and in so doing will inform the many diseases associated with misfolded proteins.